In recent years, there has been development of organic semiconducting (OSC) materials in order to produce more versatile, lower cost electronic devices. Such materials find application in a wide range of devices or apparatus, including organic field effect transistors (OFETs), organic light emitting diodes (OLEDs), photodetectors, organic photovoltaic (OPV) cells, sensors, memory elements and logic circuits to name just a few. The organic semiconducting materials are typically present in the electronic device in the form of a thin layer, for example less than 1 micron thick.
The performance of OFET devices is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance. Further requirements for the semiconducting material are a good processability, especially for large-scale production of thin layers and desired patterns, and high stability, film uniformity and integrity of the organic semiconductor layer.
Nitrogen containing small molecules, oligomers and polymers have demonstrated interesting hole transport properties.1-5 Various materials have been developed to take advantage of this physical property in organic light emitting devices (OLED), in organic field-effect transistors (OFET) and organic photovoltaic cells (OPV). However, most of those materials show poor solubility or poor structural organization in the solid state.1-5 Furthermore, these materials have generally required complex synthetic routes to yield the final material.
Therefore, there is still a need for OSC materials that are easy to synthesize, show good structural organization and film-forming properties, exhibit good electronic properties, especially a high charge carrier mobility, good processibilty, especially a high solubility in organic solvents, and high stability in air. For use in OFETs there is also a need for OSC materials that allow improved charge injection into the semiconducting layer from the source-drain electrodes. For use in OPV cells, there is a need for OSC materials having a low band-gap, which enable improved light harvesting by the photoactive layer and can lead to higher cell efficiencies.
It was an aim of the present invention to provide compounds for use as organic semiconducting materials that do not have the drawbacks of prior art materials as described above, are easy to synthesize, and do especially show good processibility, high stability, good solubility in organic solvents, high charge carrier mobility, and a low band-gap. Another aim of the invention was to extend the pool of organic semiconducitng materials available to the expert. Other aims of the present invention are immediately evident to the expert from the following detailed description.
The inventors of the present invention have found that these aims can be achieved by providing materials as described hereinafter. These materials are based on polymers comprising one or more phenanthro[1,10,9,8-c,d,e,f,g]carbazole units.
It was found that these polymers are suitable for use as OSC materials in electronic devices, especially in OFETs and OPV cells, and as charge transport layer or interlayer material in polymer light emitting diodes (PLEDs), as they have good processibility and solubility, and at the same time show a high charge carrier mobility, a low band-gap and a high oxidative stability.
In particular it was found that polymers having one or more phenanthro[1,10,9,8-c,d,e,f,g]carbazole units have similar hole transport and photovoltaic properties compared to polycarbazoles, but with improved pi-pi stacking and solid state organization.
Initially reported in 20016 poly(2,7-carbazole)s have been suggested as materials for organic electronic, particularly for OPV with power conversion efficiency <6%.7-9 Similar to this carbazole core, the synthesis of phenanthro[1,10,9,8-c,d,e,f,g]carbazole core has been recently reported.10-12 The phenanthro[1,10,9,8-c,d,e,f,g]carbazole and other similar derivatives exhibit good pi-pi stacking in the solid state.11 For example in perylo[1,12-b,c,d]thiophene, which is an isoelectronic analogue, the intermolecular packing results in a high hole mobility (0.05 cm2·V−1·s−1) in thin-film OFETs and significantly higher mobility (0.8 cm2·V−1·s−1) in single-crystal wire OFETs.13 However, there have been no reports that the phenanthro[1,10,9,8-c,d,e,f,g]carbazole unit has been successfully integrated into a polymeric structure. The low solubility of the initially reported phenanthro[1,10,9,8-c,d,e,f,g]carbazole structure limits its extension into a polymeric structure.11,12 Furthermore, the inclusion of the phenanthro[1,10,9,8-c,d,e,f,g]carbazole ladder-structure, namely the bis-N-annulated quaterrylene,10 into a polymeric structure would also result in low solubility due to the highly extended aromatic unit in the polymer backbone that would lead to aggregation.